The present invention relates to a method of forming a semiconductor device having a multi-layered structure, and more particularly to a method of forming a tungsten plug existing in either a through hole formed in a second level or higher level inter-layer insulator for eclectically connecting two different level interconnections or a contact hole formed in a first level inter-layer insulator extending over a semiconductor substrate for eclectically connecting an interconnection and a semiconductor device formed on the semiconductor substrate.
In the multi-layered structure of the semiconductor device, a metal plug is formed in a through hole formed in a second level or higher level inter-layer insulator for eclectically connecting two different level interconnections extending over and under surfaces of the second level or higher level inter-layer insulator. A metal plug is also formed in a contact hole formed in a first level inter-layer insulator extending over a semiconductor substrate for eclectically connecting a first level interconnection extending over the first level inter-layer insulator and a semiconductor device formed on the semiconductor substrate. In this specification, the term xe2x80x9cholexe2x80x9d means to include both the through hole and the contact hole. In the prior art, the metal plug comprises an aluminum plug deposited by a sputtering method. As the shrinkage of the semiconductor device has been on the increase, an aspect ratio of the hole such as the through hole or the contact hole is increased. The increase in the aspect ratio of the hole makes it difficult to deposit a metal layer within the hole by the sputtering method to form the metal plug.
In place of the sputtering method, a chemical vapor deposition method has recently been attracted due to its good step coverage. If the chemical vapor deposition method is used, tungsten as a refractory metal is deposited to fill the hole. If tungsten is deposited by the chemical vapor deposition, not only the tungsten plug is deposited within the hole in the inter-layer insulator but also an unnecessary tungsten layer is deposited over the inter-layer insulator. The unnecessary tungsten layer is then removed by a chemical mechanical polishing method.
If the chemical vapor deposition method and the subsequent chemical mechanical polishing method are carried out, then a seam is formed, wherein a cavity is formed which extends along a center line of the tungsten plug. The formation of the seam deeply depends upon the manufacturing processes for forming the tungsten plug, for which reason the conventional method of forming the tungsten plug will subsequently be described in detail by taking an example of the tungsten plug being formed in the through hole. FIGS. 1A through 1F are fragmentary cross sectional elevation views illustrative of semiconductor devices with tungsten plugs in the through holes formed in the inter-layer insulators in sequential steps involved in the conventional method of forming the semiconductor device.
With reference to FIG. 1A, a base oxide layer 1 is formed on a surface of a semiconductor substrate which is not illustrated, wherein one or more semiconductor devices are formed on the surface of the semiconductor substrate. A first aluminum interconnection layer is formed on a top surface of the base oxide layer 1. The first aluminum interconnection layer is patterned by a photo-lithography process and a subsequent dry etching process to form plural first level interconnections 2 on the top surface of the base oxide layer 1.
With reference to FIG. 1B, a first inter-layer insulator 3 is entirely deposited over the first level interconnections 2 and the base oxide layer 1 so that the first level interconnections 2 are completely buried in the first inter-layer insulator 3. A top surface of the first inter-layer insulator 3 is planarized by a chemical mechanical polishing method to form a planarized of surface of the first inter-layer insulator 3.
With reference to FIG. 1C, through holes 4 are formed in the first inter-layer insulator 3 by a photo-lithography and a subsequent dry etching process so that the through holes 4 reach top surfaces of the first level interconnections 2, whereby parts of the first level interconnections 2 are shown through the through holes 4.
With reference to FIG. 1D, a thin titanium nitride film as a barrier layer not illustrated is entirely deposited by a sputtering method, so that the thin titanium nitride film extends on the top surface of the first inter-layer insulator 3, side walls of the through holes 4 and bottoms of the through holes 4. This thin titanium nitride film serves as a barrier layer which improves an adhesiveness between silicon oxide and tungsten Subsequently, a thin tungsten core film is grown from micro crystals of tungsten by a chemical vapor deposition method on the thin titanium nitride film as a barrier layer, so that the thin tungsten core film is formed from the micro crystals of tungsten and the thin tungsten core film extends entirely on the thin titanium nitride film as a barrier layer. The micro crystals of tungsten serve as seeds or cores to be grown up and become the thin tungsten core film. In the process of forming the micro crystals of tungsten, the chemical vapor deposition process is carried out by utilizing a reduction reaction of WF6 with SiH4, wherein a reaction rate is low. When the thin tungsten core film is grown to have a thickness of about 500 angstroms, then the current growth is discontinued. This thin tungsten core film serves as a core for a further growth to a bulk tungsten layer. After the thin tungsten core film has been grown, then the used Source gases of the chemical vapor deposition is changed to WF6 and H2, so that another chemical vapor deposition is carried out by utilizing another reduction reaction of WP6 with H2, wherein the reaction rate is high so that a tungsten layer 7 is grown from the thin tungsten core film, whereby the tungsten layer 7 extends within the through holes 4 and on the top surface of the first inter-layer insulator 3. As a result, the through holes 4 are completely filled with the tungsten layer 7.
With reference to FIG. 1E, the tungsten layer 7 and the barrier layer, which extend over the top surface of the first inter-layer insulator 3, are removed by a chemical mechanical polishing method, so that the tungsten layer 7 and the barrier layer remain only within the through holes 4, whereby tungsten plugs 9 are formed within the through holes 4. Tops of the tungsten plugs 9 within the through holes 4 are leveled to the top surface of the first inter-layer insulator 3.
With reference to FIG. 1F, a second aluminum interconnection layer is formed on the top surface of the first inter-layer insulator 3 and on the tops of the tungsten plugs 9 within the through holes 4. The second aluminum interconnection layer is patterned by a photo-lithography process and a subsequent dry etching process to form plural second level interconnections 6 on the tops of the tungsten plugs 9 within the through holes 4, so that the second level interconnections 6 are electrically connected to the first level interconnections 2 through the tungsten plugs 9 within the through holes 4.
The growth mechanism of growing the bulk tungsten layer 7 from the thin tungsten core film will again be described in detail with reference again to FIG. 1D. In an initial stage of the growth, the thin tungsten core film is grown on the inside walls of the through holes 4. This makes it easy to supply the source gases of the chemical vapor deposition into spaces of the through holes 4, whereby the growth of the tungsten layer is progressed satisfactorily or smoothly. As the growth of the tungsten layer is progressed and the tungsten layers within the through holes 4 become thick, whereby the spaces of the through holes 4 are narrowed, thereby making it difficult to supply the source gases of the chemical vapor deposition into the narrowed spaces of the through holes 4. This means that if the spaces of the through holes 4 are narrowed together with the growth of the tungsten layers on the side walls of he through holes 4, then this makes it difficult to supply the source gases of the chemical vapor deposition into the narrowing spaces of the through holes 4, whereby the growth rate of the tungsten layer by the chemical vapor deposition is reduced. In a final stage of filling the through holes 4 with the tungsten layers 7, the growth rate of the tungsten layer by the chemical vapor deposition is almost zero. Namely, the micro crystal structure of tungsten in the vicinity of the center lines of the through holes 4 is like that the micro crystals are not bonded rather are in contact with each other. This means that many micro crystal defects 10 of the micro crystal of the tungsten layers 7 within the through holes 4 are formed along the center lines of the through holes 4.
The mechanism of the subsequent chemical mechanical polishing method will be described with reference again to FIG. 1E. The mechanism of the subsequent chemical mechanical polishing to the tungsten layer is that polishing particles shave oxidized tungsten by an oxidizing agent of hydrogen peroxide included in a polishing agent. Hydrogen peroxide as the oxidizing agent may, however, enter along the crystal defects 10 of the micro crystal of the tungsten layers 7 within the through holes 4, whereby the oxidation of the tungsten layer appears not only on the top surface of the tungsten layer 7 but also along the crystal defects 10 of the micro crystal of the tungsten layers 7 within the through holes 4. As a result, the oxidized tungsten not only on the top surface of the tungsten layer 7 but also along the crystal defects 10 are removed by the chemical mechanical polishing process, whereby not only the tungsten layer 7 over the top surface of the first inter-layer insulator 3 but also the tungsten layer 7 in the vicinity of the crystal defects 10 of the micro crystal of the tungsten layers 7 within the through holes 4 are removed. As a result, cavities 8 are formed along the center lines of the tungsten plugs 9 within the through holes 4. Once micro cavities have been formed, then the polishing agents also enter into the micro cavities and a contact area between the polishing agents and the tungsten layers 7 is increased, the cavities 8 in the tungsten plugs 9 are rapidly enlarged. These cavities 8 are also so called to as seams 8. Foreign matters or extraneous materials are likely to be received within the seams 8 of the tungsten plugs 9 in the through holes 4. It may be possible that a different film is formed over the tungsten layer 9 whereby a temperature is increased. In this case, not only a problem with contact failure is raised but also another problem is raised like that a moisture adhered in an inside face of the seam 8 is rapidly evaporated to cause an expansion of the film or peeling the film.
In the above circumstances, it had been required to develop a novel method of forming tungsten plugs in holes formed in an inter-layer insulator of a semiconductor device free from the above problem.
Accordingly, it is an object of the present invention to provide a novel method of forming tungsten plugs in a holes formed in an inter-layer insulator of a semiconductor device free from the above problems.
It is a further object of the present invention to provide a novel method of forming tungsten plugs in holes formed in an inter-layer insulator of a semiconductor device, wherein no seam is formed along center lines of the tungsten plugs in a chemical mechanical polishing process for forming the tungsten plugs in the holes.
It is a still further object of the present invention to provide a novel method of forming tungsten plugs in holes formed in an inter-layer insulator of a semiconductor device, wherein a yield of the semiconductor device is improved.
The first present invention provides a method of forming a tungsten layer having a high film density, which at least fills a hole of an insulation layer. The method comprises the steps of: carrying out a first chemical vapor deposition process for growing a micro crystal tungsten thin film on at least an inside face of the hole; and carrying our a second chemical vapor deposition process for growing a tungsten layer from the micro crystal tungsten thin film so that the tungsten layer at least fills the hole, wherein the second chemical vapor deposition process is carried out at a substrate temperature of not less than 475xc2x0 C. and at a growth chamber pressure in the range of 90 Torr to 150 Torr, so that the tungsten layer has a high film density and a reduced film stress.
The second present invention provides a method of forming a tungsten plug in a hole of an insulation layer The method comprises the steps of: carrying out a first chemical vapor deposition process for growing a micro crystal tungsten thin film on at least an inside face of the hole, carrying our a second chemical vapor deposition process for growing a tungsten layer from the micro crystal tungsten thin film so that the tungsten layer fills the hole and also extends over a top surface of the insulation layer; and carrying out a chemical mechanical polishing process for selectively removing the tungsten layer over the top surface of the insulation layer and leaving the tungsten layer in the hole, thereby to form a tungsten plug in the hole, wherein the second chemical vapor deposition process is carried out at a substrate temperature of not less than 475xc2x0 C. and at a growth chamber pressure in the range of 90 Torr to 150 Torr, so that the tungsten layer has a high film density.
The third present invention provides a method of forming a tungsten plug in a hole of an inter-layer insulator. The method comprises the steps of: forming at least a hole in an inter-layer insulator; forming a thin barrier layer on at least an inside face of a hole; carrying out a first chemical vapor deposition process for growing a micro crystal tungsten thin film on the thin barrier layer; carrying our a second chemical vapor deposition process for growing a tungsten layer from the micro crystal tungsten thin film so that the tungsten layer fills the hole and also extends over a top surface of the inter-layer insulator; and carrying out a chemical mechanical polishing process for selectively removing the tungsten layer over the top surface of the inter-layer insulator and leaving the tungsten layer in the hole, thereby to form a tungsten plug in the hole, wherein the second chemical vapor deposition process is carried out at a substrate temperature of not less than 475xc2x0 C. and at a growth chamber pressure in the range of 90 Torr to 150 Torr, so that the tungsten layer has a high film density.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.